More than 2100 residents in Grafton have been ordered to evacuate on Tuesday as authorities prepare for the Clarence River to hit a record peak at midday which authorities warn will cause major flooding.

The State Emergency Service issued a flood evacuation order for Dovedale and North Meadow at 8.30am, telling residents to leave their homes immediately. A further 7000 people in Grafton have been told to prepare to evacuate if conditions deteriorate further.

Authorities originally expected the Clarence River in Grafton to peak at 7.9 metres about 9am on Tuesday.

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However, that later was revised up to an expected peak of 8.1 metres at midday, which was forecast to cause major flooding of downstream rural properties.

Caroline Ortel, SES regional controller, said Grafton was facing record flood levels, which could be even higher than predicted.

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"What we're dealing with there is a flood of record," she said.

"There has never been a flood of this height in recorded history of Grafton, so for everybody who is trying to work on this and make all of the predictions, there isn't historical data to go on. The river is showing signs of dropping further upstream and that is what we have to work with at the moment."

Police urged those residents ordered to evacuate to listen to emergency services and get out as soon as possible.

"Please do not leave it to the eleventh hour, please do not ignore the warnings and try to traverse floodwaters and put the lives of SES people or the lives of police at risk trying to rescue you," a police spokesman said.

A flood evacuation warning was issued at 9.20am on Tuesday for the remaining areas of North Grafton and low-lying areas of South Grafton, affecting about 7000 people. Streets that the warning applies to in South Grafton include Bent, Ryan, Cowan, Abbot, Spring, Through, Skinner, Wharf, Armidale, Beetson, Bligh, Edward, James, Kelly, Kennedy, New, Orr, and Vere streets.

Those people have been told to prepare to evacuate when instructed to do so, and to prepare their homes by raising belongings onto tables, beds and benches.

Flash flooding from rising storm water has closed roads all over the mid north coast. Photo: Frank Redward

The heavy rain had stopped in Grafton on Tuesday morning, but isolated showers and thunderstorms were forecast for the afternoon. The temperature was expected to reach 31.

The flooding was due to heavy rain from the past few days flowing into the Clarence River upstream. At nearby Lilydale, the Clarence River peaked at 20.94 metres at midnight on Monday. That was close to the 1954 flood peak of 21 metres.

The heaviest rainfall in the Illawarra was on the escarpment, which received 150 millimetres of rain, while Wollongong received 120 millimetres, the heaviest daily rainfall total in two years.

Wind gusts of 100km/h were reported in Coffs Harbour overnight, but in Sydney the wind was not as fierce as predicted, peaking at 70km/h on Sydney Harbour.

Sydney saturated

Weatherzone meteorologist Brett Dutschke said Sydney was saturated in widespread rainfall totals of between 80 and 150 millimetres overnight, causing localised flooding in some areas, including Camden.

The wettest area of the city was Frenchs Forest, which received 161 millimetres in the 24 hours to 9am on Tuesday.

Richmond was saturated in 111 millimetres of rain, the biggest daily total in 16 years in the suburb, while Bankstown, Canterbury and Badgerys Creek recorded 100 millimetres, the heaviest in 12 years.

Mr Dutschke said many of Sydney's western suburbs received their heaviest daily rainfall totals in five years overnight.

But Mr Dutschke said by 5am on Tuesday, the heaviest rainfalls had passed for Sydney.

"It will take a little bit longer for it to ease off in the Illawarra and the South Coast. The South Coast won't get any easing until this afternoon."

Calls for help

State Emergency Service spokesman Michael Eburn said officers had responded to 2900 calls for assistance for help across NSW, mostly in the north of the state.

However that number could rise rapidly as people wake on Tuesday and assessed the damage to their properties.

"Our local units have been out in Sydney dealing with localised flooding in Camden and normal call outs, such as trees down and leaking roofs," he said.

"It has certainly been more significant in the north of NSW. We've evacuated low-lying areas of North Lismore and a couple of evacuation warnings were issued for Grafton.

"We've done 19 flood rescues, and most of those are avoidable things - people entering floodwaters despite our advice."

The low pressure system is expected to move out to sea on Tuesday, the Bureau of Meteorology said.

Dangerous surf

The Bureau of Meteorology issued a severe weather warning on Tuesday for damaging surf in the Metropolitan, Mid North Coast, Hunter, Illawarra and South Coast forecast districts.

Mr Dutschke said dangerous surf conditions were expected today and for the rest of the week, with waves of up to six metres expected to hit beaches.

A Sydney buoy, however, recorded waves up to 10 metres.

"There is a fair bit of beach erosion going on," Mr Dutschke said. "The surf is going to be dangerous for a fair while after this rain clears, probably until the end of the week. The sea is going to be pretty mucky as well."

The rain is expected to ease on Tuesday morning and clear by the afternoon, with Sydney expected to reach a maximum of 27 degrees. The rain has also eased in the north of the state, where temperatures are expected to reach the low 30s on Tuesday.

"It's going to brighten up pretty quickly after this rain clears and get quite warm," Mr Dutschke said.

However, he said there was the chance of showers and thunderstorms in coming days.

"While this rain is becoming much lighter and clearing, there is still a moderate amount of instability and it's enough to cause afternoon showers to develop for the next few days and even a few storms, in eastern parts of NSW and Queensland," he said.

Roger and Caroline Richardson go swimming near Collaroy surf life saving club every day, but despite the surf being too rough for the seasoned swimmers on Tuesday, they couldn't help getting their feet wet.

"These are some of the heaviest waves we've seen in 20 years,’’ Mr Richardson said."But the water is wonderful and warm, though you won't catch anyone swimming out there today. It's very dangerous."

Local author, Val Maudson, watching the waves crash onto the footpath, said she had seen it get much worse. She grew up in the area, where she has lived most of her life.

"I remember in the second World War, there was heavy flooding through all the houses that used to be along here.

"What was most shocking was seeing people's personal belongings, pots and pans and other things, floating in the surf."

One young body-boarder said he usually surfed but Tuesday's conditions were perfect for bodyboarding in Collaroy Pool.

"It's like a washing machine in there," he said.

"It's great fun."

Road closures

The Transport Management Centre said a number of roads and highways were closed across the state, including the Pacific Highway between Grafton and Ballina.

On Tuesday morning, the following roads were closed due to flooding:

In the state's north:- The Pacific Highway is closed between Ballina and Grafton at Shark Creek and Swan Creek. No detours are available.- The Summerland Way is closed about 11 kilometres south of Kyogle and about 10 kilometres south of Casino.- The Gwydir Highway is closed about 70 kilometres west of Grafton at Jackadgery due to a landslip.- Armidale Road is closed between Ebor and Nymboida.- The Gwydir Highway is closed at the Gibralter Ranges in Glen Innes due to fallen trees.- Bangalow Road is closed between Bangalow and Lismore.- Tweed Valley Way is closed in Murwillumbah.

In the state’s north-west:- The Newell Highway is closed between Boggabilla and Moree there is no access to the Newell Highway from Queensland.

In Sydney:- Royal National Park - Audley Road at Audley Weir closed in both directions. - Shanes Park - Stony Creek Road at Causeway closed in both directions.- Glenfield - Cambridge Avenue at Causeway closed in both directions. - Kensington - One of three south-bound lanes closed on Anzac Parade at Day Avenue.

The Wakehurst Parkway at Oxford Falls has reopened in both directions.